Exemplary embodiments of the present invention relate to a method for operating a metal detection apparatus and to a metal detection apparatus operating according to this method.
A metal detection apparatus may be used to detect metal contamination in edible goods and other products. As described in WO 02/25318, modern metal apparatuses utilize a search head comprising a “balanced coil system” that is capable of detecting all metal contaminant types including ferrous, nonferrous, and stainless steels in a large variety of products such as fresh and frozen products.
A metal detection apparatus that operates according to the “balanced coil” principle typically comprises three coils that are wound onto a non-metallic frame, each exactly parallel with the other. In a typical system, the transmitter coil located in the center is energized with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the first receiver coil is connected in series with the second receiver coil having an inverse winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity, cancel out one another in the event that the system, in the absence of metal contamination, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils, the voltage induced in each receiver coil is changed typically in the range of Nano-volts. This change in balance results in a signal at the output of the receiver coils that can be processed, amplified, and subsequently be used to detect the presence of the metal contamination in a product.
In a typical system, the signal processing channels split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known, then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the contaminants, the product, and other materials, have different influences on the magnetic field so that the resulting signals differ in phase.
The signals caused by various metals or products, as they pass through the coils of the metal detection apparatus, can be split into two components, namely resistive and reactive components, according to the conductivity and magnetic permeability of the measured object. For example, the signal caused by ferrite is primarily reactive, while the signal from stainless steel is primarily resistive. Products that are conductive typically cause signals with a strong resistive component.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g., a frequency mixer or analog multiplier circuit, generates a voltage signal that represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants may be detected. However in the event that the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component.
In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
GB2423366A discloses a metal detection apparatus that is designed to switch between at least two different operating frequencies such that any metal particle in a product will be subject to scanning at different frequencies. The frequency of operation is rapidly changed so that any metal particle passing through on a conveyor belt will be scanned at two or more different frequencies. In the event that for a first operating frequency the signal component caused by a metal particle is close to the phase of the signal component of the product and thus masked, then it is assumed that for a second frequency, the phase of the signal component caused by the metal particle will differ from the phase of the signal component of the product so that the signal components may be distinguished. By switching between many frequencies, it is expected that one frequency will provide a suitable sensitivity for any particular metal type, size, and orientation.
The drive circuit of the transmitter disclosed in GB2423366A comprises an electronically programmable logic device and a driver connected to four field effect transistors, which form a full wave bridge circuit with the transmitter coil connected across. For a particular coil system, a plurality of drive maps is stored in the electronically programmable logic device, each containing a switching sequence for the switches for a respective predetermined frequency of operation of the apparatus. With the help of stored look-up tables, the microprocessor can then select an appropriate map depending on the selected frequency of operation.
Storing, updating, and selecting appropriate drive maps for a particular coil system requires considerable efforts that increase with the number of operating frequencies. The process of adapting and optimising the system for a specific application requires updating and enhancing the drive maps. In the event that the user changes the products to be tested and expects the appearance of metal contaminants that differ in sort and dimension from contaminants measured with the previous application, a laborious optimization process needs to be performed.
GB2462212B also refers to a metal detector that contains a drive circuit comprising four switches arranged as a full bridge circuit, wherein the coil system is connected across the output of the bridge. A programmable logic device controls the switches via a plurality of drive maps stored in the programmable logic device, with each drive map containing a switching sequence for a respective predetermined frequency of operation.
Also known in the art are metal detectors such as described in U.S. Pat. No. 6,724,191 B1, to Larsen, which discloses various circuits including an H-bridge switch network and a pulse width modulated switched capacitor resonator, for simultaneously resonating at several frequencies.
U.S. Pat. No. 5,859,533 to Gasnier describes an electromagnetic tomographic emitter for operating at variable frequencies to detect subsurface characteristics.
U.S. Pat. No. 5,304,927 discloses a method and apparatus for detection of metal in food products as packages of said food products are passed through the detector on a conveyor.
An exemplary embodiment of the present invention may therefore provide an improved method for operating a metal detection apparatus that uses one or more operating frequencies as well as a metal detection apparatus operating according to this method.
Particularly, an exemplary embodiment of the present invention may provide a metal detection apparatus that may easily, quickly, and with high precision be adapted for any coil system for a specific application.
More particularly, an exemplary method may allow for adjustment of the drive current applied to the transmitter coil with reduced effort and high precision.
Other advantages of exemplary embodiments of the present invention may allow for a method for operating a metal detection apparatus that comprises a transmitter unit with a drive circuit that alternately applies two different drive voltages via a first set of two drive switches to a first tail and via a second set of two drive switches to a second tail of a transmitter coil that is coupled to a receiver coil, which is connected to the input of a receiver unit.
According to an exemplary embodiment, at least a first waveform may be generated for controlling the first set of drive switches and at least a second waveform may be generated for controlling the second set of drive switches, wherein the first and second waveform that correspond to a selected operating frequency are shifted relative to one another in order to allow a desired drive current to flow through the transmitter coil.
According to an exemplary embodiment, a drive current may easily be generated for a selected operating frequency and with a level that may precisely be adjusted. Changes and adjustments may easily be performed so that the exemplary method may allow fast changes between different modes of operation. In an exemplary embodiment, the transmitter may be changed from operating with a first operating frequency and a first level of the drive current to a second operating frequency with a second level of the drive current. For example, the drive current may be adjusted in steps of a selectable resolution to a desired level practically within a clock cycle.
In order to obtain a required phase shift between the two waveforms in an exemplary embodiment, only one waveform needs to be shifted in phase. Preferably both waveforms are shifted in opposite directions relative to one another, in order to keep the phase of the drive current constant in an exemplary embodiment.
In an example, the two waveforms provided each for the first and the second set of drive switches are preferably phase shifted by 180° relative to one another. In an exemplary embodiment, each drive switch is controlled individually with a dedicated waveform. In this way, optimal control of the drive current may be obtained, and malfunctions may be avoided.
The waveforms may be shifted in steps having a step-length that preferably corresponds to the wavelength of the operating frequency divided by an integer n.
In an exemplary embodiment, segments of the polarity “0” or “1” corresponding to the waveforms may be shifted (e.g., preferably rotated in a shift register with a clock frequency that may be obtained by multiplying the integer n with the operating frequency so that the desired waveform is generated at the output of the shift registers).
In an exemplary embodiment, segments may be provided only for a half wave and may be inverted for the second half wave, thus allowing use of shorter shift registers. The segments exiting the shift registers may be applied to an inverter, which may provide segments with inverted polarity to the input of that shift register. Consequently, segments of the first half wave and the second half wave may be cycled in the shift register with the correct polarity.
Corresponding shift registers may be set up for a selected shift resolution and may be clocked with the clock frequency that corresponds to the operating frequency multiplied by integer n.
Said integer n may be selected according to the applied operating frequency so that for a lower operating frequency a higher shift resolution may be obtained and for a higher operating frequency a lower shift resolution may be obtained.
In one exemplary embodiment, the integer n is preferably selected in the range between 512 and 64 for operating frequencies in the range of 25 kHz and 250 kHz and in the range between 128 and 16 for operating frequencies in the range of 250 kHz and 1 MHz.
In exemplary embodiments, segments may be provided with such a polarity and number that a waveform with a duty cycle of 50% is represented.
In a further exemplary embodiment, steps and means may be provided, by which the closing of the first drive switch of a set of drive switches may be avoided as long as a second drive switch of this switch set is still closed, since closing of both drive switches may lead to a short-circuit between the two drive voltages. For example, this may be advantageously achieved by inverting at least the leading segment of a group of segments of a half cycle, for which a corresponding drive switch is closed, thus delaying activation of this drive switch for a corresponding time length. Inverting one or more segments provided in the shifted registers may be done with little effort. In an exemplary embodiment, the delay may correspond to the number of the inverted segments and their time length, which may correspond to a period of the clock frequency provided to the shift registers. In another exemplary embodiment, the delay may be obtained by using a counter, which in response to an input signal may provide an output signal after a predetermined number of clocks of the clock frequency have been counted. In an exemplary embodiment, the counter may be programmable so that for each operating frequency a predetermined number of clocks and thus a predetermined delay may be selected.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.